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Paternal effort is high in some monogamous mating systems. Trivers' (1972) model predicts that high male investment in brood care should evolve only when males have a high certainty of paternity. For this study, we chose two woodpecker species: the great spotted woodpecker (Picoides major) and the middle spotted woodpecker (Picoides medius). Both species were socially monogamous despite a very high breeding density in the study area. We used DNA fingerprinting to determine whether these two species were also genetically monogamous. We found that in great spotted and middle spotted woodpeckers paternal effort was high. Multi-locus DNA-fingerprinting showed that its actual paternity was also very high. In P. major all 161 young from 36 broods and in P. medius all 61 young from 13 broods were sired by the male feeding at the nest hole. There were also no cases of intraspecific brood parasitism or quasi parasitism (P. major: 114 chicks from 24 broods; P.medius: 33 chicks from 8 broods). We further found no case of mate switching during the fertile period of the female. Great spotted and middle spotted woodpeckers are typical of a group of monogamous nonpasserine birds with high male investment in brood care having low frequencies of EPP. We did not find efficient paternity guards. High certainty of paternity may be explained by paternal care being essential for female reproductive success, as in many seabirds and birds of prey. Females rarely engage in extra-pair copulations probably because they are constrained by male care. Males in both species spend little effort in acquiring mates as well as in extrapair copulations. They expend their reproductive effort in defending territories and in parental care. Females compete intensely with members of their own sex for pair formation before the time of frequent copulation. Choosing and securing a high quality partner is the only possibility to achieve high reproductive success for both sexes.